The Games Room
Eleven of them. No accounts, no ads, nothing tracked, and every one works with the aeroplane mode on.
The ones marked four letters are played by more than one person on more than one phone. One of you opens the room and reads out four letters; everybody else types them. That is the whole ritual — no app, no invite link, no sign-in. And every one of them keeps working when whatever mints those four letters is unreachable: there is a camera-and-paste handshake underneath that needs no server at all.